The present invention relates to a fast roaming system whereby a mobile terminal, while communicating with an access point, serving as a parent station, over a wireless LAN that conforms to the IEEE 802.11 specifications developed by the Institute of Electrical & Electronics Engineers (IEEE), can be quickly switched from the parent station to an adjacent access point having an overlapping communication range. The present invention relates in particular to a fast roaming system wherein, without communication being interrupted, a roaming destination access point providing better environmental conditions can be selected and a roaming completion ratio improved, and wherein a fast roaming operation can be effectively performed under a variety of conditions.
Generally, the IEEE 802.11 specifications, which constitute a well known wireless LAN technique, are typically applied for a wireless LAN.
There are two types of wireless LANs: one is a so-called AdHoc or independent connection network wherein mobile terminals are interconnected; and another is an infrastructure connection network wherein multiple mobile terminals are connected to an access point (hereinafter referred to simply as an AP) that serves as a parent station, and communicate with each other through the parent station.
In the environment inherent to an infrastructure connection network, wherein either multiple APs are adjacently located or the AP communication zones differ, the connection of a mobile terminal, which is moving during the communication process, must be transferred from a parent station AP to which currently connected to another peripheral AP. In general, the roaming system of the invention can be applied for the switching of AP connections, including switching between communication providers. For convenience, during the explanation of this invention, an AP currently connected to a mobile terminal is called a parent station AP, and an AP for which AP information can be obtained by performing an AP search, i.e., a searchable AP, is called a peripheral AP.
For a roaming operation, a process called scanning is employed to find a roaming destination AP by examining connectable peripheral APs.
To perform this scanning process, two methods are available. According to the first, a passive scanning operation is performed to receive a beacon (synchronization) signal that is periodically transmitted by an AP in order to synchronize mobile terminals with the AP. And according to the second, an active scanning operation is performed wherein a mobile terminal transmits a search packet, called a probe, to an unspecified AP and receives a response packet in return.
Generally, for roaming, the active scanning operation is performed for all, or for predesignated, wireless channels, and based on a response, a peripheral roaming destination AP is selected as the next parent station AP.
In accordance with the well known IEEE 802.11b specifications, in Japan, fourteen channels are employed in a frequency band ranging from 2.412 GHz to 2.483 GHz. Thus, when an active scanning operation, or a passive scanning operation, is performed to select a peripheral AP, a search must be made of all fourteen channels, or of predesignated channels.
During a roaming operation, when a mobile terminal is moved and the reception level for the currently connected parent station AP falls below a predetermined threshold value, passive scanning is performed to determine whether other connectable APs are present in the vicinity. When connectable peripheral APs are found, a reassociation request is issued to the one providing the highest reception level, and the mobile terminal is connected to this AP, as the new parent station AP. The roaming operation is thus completed.
Further, according to the IEEE 802.11 specifications, roaming standardization is performed for specifying the procedures whereby, when the roaming operation has been completed, the destination parent station AP notifies other APs that the mobile terminal has performed the roaming operation.
Conventionally, this type of fast roaming system is disclosed in JP-A-2001-94572 or JP-A-2002-26931, for example.
This system will now be described while referring to FIG. 1. A parent station AP 102-A, to which a station, a mobile terminal 101, is connected, fetches over a LAN 5 hopping data for APs 102-B and 102-C and stores these data. Similarly, the other APs 102 connected to the LAN 5 also fetch the hopping data for the other APs 102 over the LAN 5 and store these data. Therefore, the mobile terminal 101 can download, from the connected parent station AP 102-A, the hopping data for the peripheral APs 102-B and 102-C. That is, based on the hopping data for the peripheral APs 102-B and 102-C, which are obtained from the parent station AP 102-A, the mobile terminal 101 can store the latest wireless communication states of the peripheral APs 102-B and 102-C in a database.
On the other hand, the mobile terminal 101 may examine the quality of a beacon signal received from the parent station AP 102-A, and begin the roaming operation when the quality falls below a predetermined threshold value.
While referring to FIG. 2 as well as FIG. 1, before beginning the roaming operation, the mobile terminal 101 examines a database (step S101) to determine whether the latest wireless communication states of the peripheral APs 102-B and 102-C have been obtained from the parent station AP 102-A and stored.
When the decision is YES at step S101, i.e., the data are present in the database, the communication states in the database are compared (step S102), and a peripheral AP providing the best wireless environment, e.g., the AP 102-C, is selected and the roaming operation is performed (step S103). When the roaming operation has been completed (YES at step S104), the processing is terminated thereafter.
When the decision at step S101 is NO and the wireless communication states of the peripheral APs have not been obtained, based on the hopping data, the mobile terminal 101 scans the channels for all the APs (step S105) and obtains their wireless environments, and program control is shifted to step S103. When the decision is NO at step S104 and the roaming operation has not yet been completed, program control is returned to step S103 for the selection of the one of the remaining APs that provides the best wireless environment.
With this configuration, according to JP-A-2001-94572, when the roaming operation start condition has been established, the mobile terminal employs the downloaded data to directly provide synchronization with the peripheral APs, and performs a roaming operation to select the AP having the largest RSSI (Received Signal Strength Index). According to JP-A-2002-26931, the mobile terminal examines the obtained data to select the AP providing the best wireless environment.
With this configuration, before starting the roaming operation, the mobile terminal obtains, from a parent station AP, the hopping data for the peripheral APs or received signals, and compares these data to select the AP providing the best wireless environment, and then performs the roaming operation. Therefore, the roaming speed can be increased.
In the conventional fast roaming system described above, a currently connected parent station AP receives, from peripheral APs, information to serve as a database for the selection of a roaming destination AP, and this information is only periodically fetched by the mobile terminal. Thus, the available information tends not to be current. Accordingly, it is unfavorable that, for the mobile terminal, the database contents used when selecting a roaming destination AP frequently differ from the current ones.
Further, in the conventional fast roaming system described above, the AP data collected by each AP is limited to that for APs located around the periphery of the pertinent AP and that the thus collected data include no condition information that indicate whether a mobile terminal can be connected. Therefore, it is unfavorable that there is high probability that a connection-disabled AP will be selected as a roaming destination AP by the mobile terminal.